Fracture Surface Of Concrete Research Articles

Efficient Evaluation of Concrete Fracture Surface Roughness Using Fringe Projection Technology.

The evaluation of concrete surface roughness is crucial in the field of civil engineering. The purpose of this study is to propose a no-contact and efficient method for the measurement of the roughness of concrete fracture surfaces based on fringe-projection technology. A simple phase-correction method using one additional strip image is presented for the phase unwrapping to improve the measurement efficiency and accuracy. The experimental results indicate that the measuring error for plane height is less than 0.1mm, and the relative accuracy for measuring a cylindrical object is about 0.1%, meeting the requirements for concrete fracture-surface measurement. On this basis, three-dimensional reconstructions were carried out on various concrete fracture surfaces to evaluate the roughness. The results reveal that the surface roughness (R) and fractal dimension (D) decrease as the concrete strength increases or the water-to-cement ratio decreases, consistent with previous studies. In addition, compared with the surface roughness, the fractal dimension is more sensitive to the change in concrete surface shape. The proposed method is effective for detecting concrete fracture-surface features.

Effect of Leaching Behavior on the Geometric and Hydraulic Characteristics of Concrete Fracture.

The leaching of material from concrete fracture surfaces has an impact on the structural concrete in service, but the number of studies that consider the effect of the coupling of the leaching, fracture geometry and hydraulic processes on concrete fractures is insufficient. In this study, a series of experiments was conducted, and a leaching model proposed, to investigate the mechanism of leaching behavior on the geometric and hydraulic characteristics of concrete fractures. Following the leaching experiment, the evolution of fracture geometric characteristics was observed by a three-dimensional (3D) laser scanning technique, finding that the fracture produces residual leached depth and local uneven leaching, which results in a decrease in roughness. The hydraulic characteristics were then investigated by permeability tests, and it was found that the fracture hydraulic aperture and permeability increase monotonically with leaching time. A simulation of fluid flow in a numerical fracture revealed the effect of residual leached depth and a decrease in roughness on the hydraulic characteristics. Finally, based on the analysis of the chemical composition of the leaching solution, a leaching model of concrete rough fracture surface is proposed and the mechanism of leaching behavior is discussed. These new findings are useful for the understanding of the development of leaching, local to concrete fracture surfaces.

The fracture behavior and microstructure of calcium aluminate cement concrete with various water-cement ratios

The present paper is aimed to examine the fracture behavior of calcium aluminate cement concrete. A total of 90 notched beams with water-cement ratios (W/C) between 0.3 and 0.55 were selected and subjected to the three-point bending tests. Two methods, i.e. the size effect method (SEM) and the work of fracture method (WFM), were adopted to calculate the fracture parameters for all the mix designs. As shown by the obtained results, a decrease in the W/C ratio from 0.55 to 0.3 caused a linear increase in the fracture toughness. In this condition, the brittleness number increased by around 50%. In addition, the results also indicated that the maximum fracture energies, Gf and GF, obtained from the SEM and WFM, respectively, were seen at W/C = 0.4 and also the characteristic length, lCh, in the WFM and the effective length of the fracture process zone, Cf, in the SEM reduced. Furthermore, the concrete fracture surface became somehow smoother due to better cohesion between the cement paste and aggregate. Furthermore, the findings also confirmed the existence of an approximate correlation between the fracture energy obtained by the WFM and the one by the SEM (GF=2.28Gf).

A Fractal Model of Cracking of Cement Matrix Composites

The modern methods of materials (including cement matrix materials) design and testing impose the application of an approach appropriate to materials engineering. A quantitative description of the association between the properties of these materials and their structure is a necessity. What remains the scientific aim, however, is the clarification and description of the occurring phenomena by means of models mapping their actual behavior in the closest way possible. The article presents a cracking fractal model based on tests on the morphology of concrete fracture surfaces. The recorded fractal nature of the cracking of cement matrix materials enabled fractal geometry in the model development to be applied. Owing to the application of statistical analysis, together with an extensive base of data on the profile lines separated out of the real fracture surfaces of concrete, it was possible to develop a cracking fractal model. Not only does this model satisfy the condition of the equality of the fractal dimension of the real and model profile lines, it also offers the possibility of introducing an order to the apparently chaotic phenomena, such as the cracking process. An advantage and novelty of the model is that unlike the other authors’ proposals, there is a possibility of reaching an infinitely large number of solutions for model profile lines, which approximates the model to the real-life scenario. The results of fractal tests were supplemented with strength measurements, identifying concrete’s compressive and fracture toughness (determining the critical stress intensity factor KIcS). A connection between the fractal dimension and the investigated properties of concrete was demonstrated. A higher fractal dimension was observed in the profile lines separated out of the fracture surfaces of concretes of higher water–cement ratio. The advantages of the model include the simplicity and applicability in model studies on other materials of the cement matrix.

The influence of various loading rates on concrete fracture surface areas

The fracture surface areas of concrete prisms that were tested in bending under various loading rates are presented in this paper. Loading rates corresponding to quasi-static, impact and blast were applied to cover a wide range of load action that may be applied on a concrete structure over its lifetime. It was verified experimentally that the fracture surface tends to decrease with increasing loading rate from quasi-static to hard impact. However, fracture surface gained under the blast experiments do not follow this trend as there are probably other factors governing the response to the blast. It seems that the inertia or the stress concentration at the crack tip are the governing factors for the dynamic increase factor (DIF) and less for the fracture surface, while the different physical characteristics of the testing method for blast versus quasi-static or impact loading may also influence the results.

СВЯЗЬ ФРАКТАЛЬНОЙ РАЗМЕРНОСТИ ПОВЕРХНОСТИ РАЗРУШЕНИЯ БЕТОНА С ХАРАКТЕРИСТИКАМИ МЕХАНИКИ РАЗРУШЕНИЯ

As it was found previously, the concrete fracture surface formed from tensile force is described by fractal geometry methods. It is shown thatthe fractal dimension value is related to the tensile stress gradient φ_i, to the aggregate size and, as shown earlier, does not depend on the strength of concrete. Moreover, the fractal dimension depends on the size of the sample only until its size reaches a value to which linear fracture mechanics is applicable. The stress intensity factor is related to the fractal dimension, and both characteristics are related to the aggregate size. A connection for the critical stress intensity factor K_Ic^f(l,φ_i) characterizing the crack resistance of the material in nonlinear fracture mechanics with the crack size l and the specimenis proposed. The stress intensity factor for a fractal crack K_Ic^f(l,φ_i) can be used to calculate structures using nonlinear fracture mechanics.

Applications of Steel Slag Powder and Steel Slag Aggregate in Ultra‐High Performance Concrete

The applications of steel slag powder and steel slag aggregate in ultra‐high performance concrete (UHPC) were investigated by determining the fluidity, nonevaporable water content, and pore structure of paste and the compressive strength of concrete and by observing the morphologies of hardened paste and the concrete fracture surface. The results show that the fluidity of the paste containing steel slag is higher. The nonevaporable water content of the hardened paste containing steel slag powder is close to that of the control sample at late ages. Both steel slag powder and steel slag aggregate react and connect tightly to gels and hardened paste, respectively. When the cement replacement ratio is no more than 10%, the proportion of pores larger than 50 nm in the hardened paste containing steel slag powder is close to that of the control sample, and the UHPC containing steel slag powder can display satisfactory compressive strengths. The UHPC containing steel slag aggregate demonstrates higher compressive strengths.

The effect of water to cement ratio on fracture parameters and brittleness of self-compacting concrete

The paper describes an experimental research on fracture characteristics of self-compacting concrete (SCC). Three point bending tests conducted on 154 notched beams with different water to cement (w/c) ratios. The specimens were made from mixes with various w/c ratios from 0.7 to 0.35. For all mixes, common fracture parameters were determined using two different methods, the work-of-fracture method (WFM) and the size effect method (SEM). Test results showed that with decrease of w/c ratio from 0.7 to 0.35 in SCC: (a) the fracture toughness increases linearly: (b) the brittleness number is approximately doubled: (c) the effective size of the process zone cf in SEM and the characteristic length (lch) in WFM decrease which may be explained by the change in structural porosity of the aggregate–paste transition zone; and (d) the fracture surface of concrete is roughly smoother, which can be attributed to the improved bond strength between the aggregates and the paste. Also, the results showed that there is a correlation between the fracture energy measured by WFM (GF) and the value measured through SEM (Gf) (GF≅2.92Gf).

Study on Fractal Elastic Damage Constitutive Law for Concrete Material under Static Load

The stochastic properties and discreteness of macroscopic property for concrete appear on mechanical property and fracture surface. In consideration of stochastik and discreteness of fracture surface, a class of mesoscopic damage mechanics model of concrete based on spring model, are put forward to understand the real damage evolution characteristics of concrete at the level of constitutional law. A kind of spring-slipper model is introduced to reflect the elastic-plastic damage behavior. It has been confirmed that fracture surface of concrete has self-affine fractal characteristic only on a certain spatial scale, but the actual fracture surface of concrete is a stochastic surface with multi-fractal characteristics. Uniaxial test was operated, combined with the Computerized Tomography test of concrete, to study the evolution of crack surface from mesoscopic level to macroscopic level. Compared with the existing damage constitutive law and experimental results preliminarily, the feasibility of fractal damage constitutive law is verified.

Fracture analysis of concrete using singular fractal functions with lattice beam network and confirmation with acoustic emission study

In the present study singular fractal functions (SFF) were used to generate stress–strain plots for quasi-brittle material like concrete and cement mortar and subsequently stress–strain plot of cement mortar obtained using SFF was used for modeling fracture process in concrete. The fracture surface of concrete is rough and irregular. The fracture surface of concrete is affected by the concrete’s microstructure that is influenced by water cement ratio, grade of cement and type of aggregate [1–4]. Also the macrostructural properties such as the size and shape of the specimen, the initial notch length and the rate of loading contribute to the shape of the fracture surface of concrete. It is known that concrete is a heterogeneous and quasi-brittle material containing micro-defects and its mechanical properties strongly relate to the presence of micro-pores and micro-cracks in concrete [1–4]. The damage in concrete is believed to be mainly due to initiation and development of micro-defects with irregularity and fractal characteristics. However, repeated observations at various magnifications also reveal a variety of additional structures that fall between the ‘micro’ and the ‘macro’ and have not yet been described satisfactorily in a systematic manner [1–11,15–17]. The concept of singular fractal functions by Mosolov was used to generate stress–strain plot of cement concrete, cement mortar and subsequently the stress–strain plot of cement mortar was used in two-dimensional lattice model [28]. A two-dimensional lattice model was used to study concrete fracture by considering softening of matrix (cement mortar). The results obtained from simulations with lattice model show softening behavior of concrete and fairly agrees with the experimental results. The number of fractured elements are compared with the acoustic emission (AE) hits. The trend in the cumulative fractured beam elements in the lattice fracture simulation reasonably reflected the trend in the recorded AE measurements. In other words, the pattern in which AE hits were distributed around the notch has the same trend as that of the fractured elements around the notch which is in support of lattice model.

A method to evaluate the three-dimensional roughness of fracture surfaces in brittle geomaterials

Conventionally, the evaluation of fracture surface roughness in brittle geomaterials, such as concrete and rock, has been based on the measurement and analysis of two-dimensional profiles rather than three-dimensional (3D) surfaces. The primary reason for doing so was the lack of tools capable of making 3D measurements. However, in recent years, several optical and mechanical measurement tools have become available, which are capable of quickly and accurately producing high resolution point clouds defining 3D surfaces. This paper provides a methodology for evaluating the surface roughness and roughness anisotropy using these 3D surface measurements. The methodology is presented step-by-step to allow others to easily adopt and implement the process to analyze their own surface measurement data. The methodology is demonstrated by digitizing a series of concrete fracture surfaces and comparing the estimated 3D roughness parameters with qualitative observations and estimates of the well-known roughness coefficient, R(s).

The necessary number of profile lines for the analysis of concrete fracture surfaces

The article describes a technique for the measurement of the level of complexity of fracture surfaces by the method of vertical sections, and a performed statistical analysis of the effect of profile lines on the fractographic and fractal parameters of fractures, i.e. the profile line development factor, RL, and the fracture surface development factor, RS, (as defined by the cycloid method), as well as the fractal dimension, DC, (as determined by the chord method), and the fractal dimension, DBC, (as determined by the box method). The above-mentioned parameters were determined for fracture surfaces of basalt and gravel concretes, respectively, which had previously been subjected to fracture toughness tests. The concretes were made from mixtures of a water/cement ratio ranging from 0.41 to 0.61 and with a variable fraction of coarse aggregate to fine aggregate, Cagg./Fagg., in the range from 1.5 to 3.5. Basalt and gravel aggregate of a fraction to maximum 16 mm were used to the tests. Based on the performed analysis it has been established that the necessary number of concrete fracture profile lines, which assures the reliability of obtained testing results, should amount to 12.

Extension of FCM to plain concrete beams with vertical tortuous cracks

Fictitious crack model has been appropriately modified to make it applicable for plain concrete beams with vertical tortuous cracks. A number of tortuous cracks are generated considering the crack deviations as random variable based on the maximum aggregate size. Instead of a straight-line crack in FCM a number of simulated tortuous cracks are replaced to model the roughness of fracture surface of concrete. Plain concrete beams with such tortuous cracks are analyzed to study the effect of the tortuousity of the cracks on the various fracture parameters.

A fractal fracture model and application to concrete with different aggregate sizes and loading rates

Recent developments in fractal theory suggest that fractal may provide a more realistic representation of characteristics of cementitious materials. In this paper, the roughness of fracture surfaces in cementitious material has been characterized by fractal theory. A systematic experimental investigation was carried out to examine the dependency of fracture parameters on the aggregate sizes as well as the loading rates. Three maximum aggregate sizes (4.76 mm, 12.7 mm, and 19.1 mm) and two loading rates (slow and fast loading rate) were used. A total of 25 compression tests and 25 tension tests were performed. All fracture parameters exhibited an increase, to varying degrees, when aggregates were added to the mortar matrix. The fracture surfaces of the specimens were digitized and analyzed. Results of the fractal analysis suggested that concrete fracture surfaces exhibit fractal characteristics, and the fractal geometry provide a useful tool for characterizing nonlinear fracture behavior of concrete. Fractal dimension D was monotonically increased as maximum aggregate sizes increase. A new fractal fracture model was developed which considers the size and shape of aggregate, and the crack paths in the constituent phases. Detailed analyses were given for four different types of fracture paths. The fractal fracture model can estimate fractal dimension for multiphase composites.

Influence of concrete composition on the characterization of fracture surface

An experimental method based on laser triangulation was used to measure the 3-D profile of fracture surface of concrete. The projective-covering method was established to determine the fractal characteristics of fracture surface. Based on the experiment, the influence of composition of concrete on the fractal dimension, including water–binder ratio, maximum aggregate size and aggregate type, was investigated. The variation of fractal dimension with the material composition was also analysed to make further understanding of fracture mechanism of concrete.

On the characterization of polymer concrete fracture surface

Fracture surface geometry is often related to material toughness. This approach requires a quantitative description of the fracture surface. The aim of this work was to investigate scale effects on the geometrical features of the fracture surface of polymer concrete (cementless concrete) with different microstructures. Polymer concrete (PC) is a concrete-like composite, in which a resin binder substitutes the cement binder. The fracture surfaces were characterized at different magnifications by: fractal dimension, surface roughness ratio and profile roughness ratio. Relationships between parameters describing the geometry of PC fracture surface were analyzed. The main conclusion is that the geometry of the PC fracture surface depends on the scale of observation.

Reconstruction and analysis of 3-D profile of fracture surface of concrete

A new experimental technique based on laser triangulation to analyze and reconstruct 3-D fracture surfaces of concrete was proposed. Results of the reconstruction of a standard object and of the fracture surface of a concrete specimen showed that this technique has reasonable accuracy. Compared with the traditional analysis methods, this method is very convenient to determine the fractal dimension and other parameters of fracture surfaces, since indirect or destructive experimental procedures that are laborious and time-consuming are avoided. Quantitative analysis of roughness and the fractal feature of the fracture surface of concrete was also carried out. According to the value of roughness parameter R s (i.e. the ratio of actual surface area to projected surface), it is shown that the true fracture surface area is about 45.4% greater than the nominal fracture area. Two methods of calculation of fractal dimension, i.e. modified slit-island technique and vertical section analysis, were employed and preliminary analysis results indicated that the fracture surface of concrete is fractal, with fractal dimension ranging from 2.1 to 2.2. This new technique may be of great help to precisely measure fracture parameters and eventually improve our understanding of failure mechanisms of concrete.

Rupture probability of coarse aggregate on fracture surface of concrete

In this work, the fracture surface of concrete was analyzed by the digital image analysis (DIA) technique, and the rupture probability of the coarse aggregate (RPCA) was used to represent the failure mode of the aggregate at the fracture surface. The relationships between the RPCA, the water-binder ratio (W/B), and the size and type of coarse aggregate were investigated. Preliminary results showed that: (1) RPCA increases with decreasing W/B of concrete. (2) RPCA of concrete with coarse aggregates having a maximum size of 16 mm is higher than that with other sizes of coarse aggregate. The influence of the size of coarse aggregate on the RPCA is more significant in high strength concrete than in normal strength concrete. (3) With reduction of W/B, the interfacial bond of concrete with crushed gravel improves more significantly than with round gravel according to their RPCA. (4) RPCA depends not only on the intrinsic strength, size, and shape but also on the reactivity of the coarse aggregate.

Multifractal nature of concrete fracture surfaces and size effects on nominal fracture energy

Experimental evidence of the fractality of fracture surfaces has been widely recognized in the case of concrete, ceramics and other disordered materials. An investigationpost mortem on concrete fracture surfaces of specimens broken in direct tension has been carried out, yielding non-integer (fractal) dimensions of profiles, which are then related to the ‘renormalized fracture energy’ of the material. No unique value for the fractal dimension can be defined: the assumption of multifractality for the damaged, material microstructure produces a dimensional increment of the dissipation space with respect to the number 2, and represents the basis for the so-called multifractal scaling law. A transition from extreme Brownian disorder (slope 1/2) to extreme order (zero slope) may be evidenced in the bilogarithmic diagram: the nominal fracture energyGF increases with specimen size by following a nonlinear trend. Two extreme scaling regimes can be identified, namely the fractal (disordered) regime, corresponding to the smallest sizes, and the homogeneous (ordered) regime, corresponding to the largest sizes, for which an asymptotic constant value ofGF is reached.

Assessment and evaluation of fractal dimension of concrete fracture surface digitized images

Recently, a new non-destructive technique was introduced and tested by the authors [1,2] for the evaluation of fractal dimension of fractured concrete specimens. The objectives of this paper are to expand and eleborate on the advantages of the technique, to provide a basic understanding of fractal geometry and to discuss the potential application of fractal geometry to fracture in cementitious materials. This may eventually lead to a better understanding of the fracture mechanics properties of such materials. Through this technique and the utilization of an Image Analyzer System, the fracture surface was photographed and stored as an image. Different methods of fractal analysis, e.g. modified slit island technique (SIT) and two-dimensional Fourier spectral analyses (2D FFT) were employed. Fractured concrete specimens with a maximum aggregate size of 37.5 mm and a projected fracture area of 367.5 mm long and 125 mm wide were used in the study. Results of the analysis suggest that concrete fracture surface exhibit fractal characteristics over the scales considered. A scaling relation between roughness at different length scales holds only over a limited range of wave lengths. The mean fractal dimension for the modified slit island technique is D=2.21 and the mean stochastic fractal dimension for two-dimensional Fourier spectral analyses is D=2.59. Also, the digitized fracture surface images were found to mimic the actual fracture surfaces.